Definitely fun.Thanks [MENTION=16159]Bold Seagull[/MENTION] - that is very thorough. Sounds like fun!
A good rule of thumb would be the longer the event the more 'seriously' it's taken.
A 20/40/60 mile event is more likely to have a bigger range of cyclists and abilities.
If you rock up at a 60/80/100 mile event be prepared to be dazzled by proper bike porn.
As the chaps say you may encounter the odd 'warrior' but 90+% of the people who are wannabe pros will look at a newbie and think 'happy days'.
One way to feel more the part would be to not turn up wearing your Sky mesh skinsuit, on a bike with stabilisers.
Think JCL's in a Man Utd kit.